Watch Lume

Too often we take it for granted that our sports watches make use of luminescence, which allows us to read the time perfectly in low and no lighting conditions, however, this wasn’t always the case. History shows getting us to this point in time was no easy feat and it did indeed cost lives too. As technology has evolved so has the legibility of the lume in our wristwatches and over the many years, a variety of materials and chemicals have been used too, though the first type of solutions were first seen in the early 20th century.

The beginnings of luminous watches

Early methods of applying luminescence to wristwatches, mainly involved mixing the radioactive material radium with zinc sulphide. However, it has been known since the late 19th century that Radium alone, in the correct dose, possesses some luminescent properties, allowing it to glow blue. But alone this wasn’t enough to emit the bright glow required and of course too much would make it harmful. Nonetheless, combining the two materials not only remedied the radiation issue but allowed for a much brighter glow. With the radioactive properties of the radium triggering the phosphor in the zinc sulphide, this made the material glow but the amount of radium required to do this also meant it wasn’t harmful to the wearer. Varying mixtures of zinc sulphide with other compounds meant that the colour of the luminescent could be changed too.

Vintage Panerai Radiomir, Image: Auctionata

Tragic history

Though the amount of radiation from the radium used on watch dials and hands were not harmful to the wearer, it did indeed have an adverse effect on those who applied the material onto these aspects of the watch. You see, back in the days the process was most commonly performed by women who would lick their paint brushes to form a fine point to paint the small markers of dials and hands – in addition to painting their fingernails and putting radium in their hair. Unbeknownst to them, they were slowly swallowing radium and over time this lead to a variety of illnesses. Unfortunately, some even died as a result of this, as they were recklessly told the material was not harmful and thought nothing of their actions. In 1927 a lawsuit was filed on behalf of these women, however sadly most of them were too ill to attend once it had gone to trial. Nonetheless, the court ruled in favour of these women – who later on became known as the Radium Girls.

Over the many years of its use, the detrimental effects of radium became more apparent and its use in watches was dramatically reduced to a point where a wristwatch in the 60’s only had 1/100th of the amount of radium used in the early 20th century. By 1968, the use of radium in watches had been forbidden, which now meant an alternative material was required to be used for the lume. Tritium became the material of choice and it was used in the same manner as radium had been before; mixing it with zinc sulphide.

However, the difference was in the lifespan of tritium in comparison to radium, with it only having a half-life of around 12-years as opposed to the 1,600 year of life of radium. That said, most watches from the 1960s, such as Rolex sports watches develop quite a desirable patina (a yellow hue), which is owed to the tritium aging. And while tritium is of course less harmful than radium, there were still health concerns, which meant the use of these materials had to be highly visible. Dials using tritium typically displayed a “T” i.e. T-SWISS-T or SWISS-T <25.

By the 1990s tritium was phased out in favour of non-radioactive materials to produce luminescence, such as photoluminescence. A material that wasn’t self-illuminating but rather required a charge from an external light-source to activate its glow. However, even this proved to have its problems as this glow would slowly fade, whereas radioactive lume omitted a more constant glow.

Today we mostly see a photoluminescent substance that goes by the name of LumiNova. This material was invented by the Japanese company Nemoto & Co. Ltd. in 1993, who later joined forces with the Swiss company RC TRITEC Ltd. to form LumiNova AG Switzerland who distribute the material. LumiNova and Super-LumiNova paints are of course in the, non-radioactive, photoluminscent category of lumes. They glow brightly after charge or exposure to a light source and fade slowly over time. The pigments are not yet known to discolour with age and are very fragile and can be affected by humidity, causing it to flake or in some cases crumble. Other companies use similar compounds, like Seiko’s Lumibrite and Rolex Chromalight, which maintains the same properties as Super-LumiNova.

Though the materials and process for creating lume for watches has changed over the years the requirement and uses haven’t. Whether used for diving, working at night or even just checking the time in lowlight situations, your watch will provide the luminance you need when called upon, so I think it’s about time we started appreciating our lume.